A Parametric Evaluation of Multi-Webbed Composite Joists Based on EC5

Abstract The introduction of engineered beams offers long span, low cost and light weight joists in comparison to traditional solid timber sections. However, the vibration criteria in the design codes may significantly reduce the permissible span of the joists for domestic construction. This paper investigates the influence of geometric and material variability on permissible spans of a series of multi-webbed composite joists, which are designed in accordance with the requirements of Eurocode 5 and its UK National Annex. Geometric variability comprises four different profiles, namely; I-beams, Double I-beams, Box beams and Box I-beams/joists. The variations between the profiles are web-flange connection details and the number of webs, which vary from one to three depending on the profile. Material variability involves two different web materials, OSB and plywood, which are considered together with two different flange materials, C16 and C24. This study shows that serviceability would in general govern design requirements in UK timber flooring systems. Out of four different sub-criteria defined under the serviceability conditions, the deflection under unit point load is the governing condition in most cases. It was found that the correct choice of materials and beam profile can enhance the permissible span by up to 34%. When lateral restraint is not provided, lateral stability becomes the controlling criterion as beam depth increases. An inverse relationship has been found between the permissible span and the beam depth for an I-beam profile, whereas for the Double I-beams a constant permissible span was observed for varied beam depths. For the Box beams and Boxed I-beams, the permissible span will increase in proportion to beam depth. This study also shows that the effect of lateral stability on solid timber joists is less pronounced than for engineered beams.